The present invention relates in general to a process for fabricating coated glass tubes, especially a fiberoptic waveguide preform by a plasma pulse-induced chemical vapor deposition (PICVD) process and, more particularly, to a process in which a gas stream is passed through a glass tube, and a series of layers of specified thicknesses and of a specific refractive index profile are deposited from the gas stream within a coating region on the inside of the glass tube. The invention also relates to tubular glass blanks having a geometry especially useful for the production of such preforms.
In the PICVD process, for example, as described in EP 0 036 191, short plasma pulses in the low pressure range are used to deposit thin dielectric layers on the inner surface of a glass tube in an axial coating region from a mixture of reaction gases flowing through the tube. The formation of layers is carried out with each plasma pulse virtually simultaneously over the entire section of the tube to be coated. During the breaks between pulses, the tube section is again filled with fresh reaction gas.
The thickness of the layer deposited by a plasma pulse at a particular point in the glass tube is proportional to the density of the layer-forming molecules at that point--before the plasma pulse--each molecule participating in the coating reaction, which thus has a yield of 100%. Insofar as the plasma-generating field has a constant azimuth (axially symmetrical), the thickness of the layers is nevertheless independent of the azimuth because of the circular symmetry of a tube.
Since in a glass tube traversed by a gas there is a pressure drop, i.e., a negative pressure gradient, of the layer-forming gases along the regions of the glass tube being coated, the density of the layer-forming molecules decreases in the direction of gas flow, resulting in a decrease of the rate of coating in the direction of gas flow. Generally, however, a constant layer thickness over the entire coating region is sought. This is particularly the case for internally coated glass tubes from which preforms for fiberoptic waveguides are produced. These preforms are conventionally produced by collapsing the coated tube into a rod, and high standards must be met for the uniformity of the thicknesses of the layers in such a rod.
To achieve a uniform thickness of the deposited layers over the entire coating region, it has been proposed to insert into the glass tube a conical glass rod extending coaxially along the region to be coated (British Patent No. 2,079,267). In this process, it is intended that the periphery of the glass rod be largest at the gas-entry end and smallest at the gas-exit end. This process, however, has the disadvantage that the glass rod is also coated. In addition, the glass rod must be mounted at both ends of the tube to be coated so precisely that deviations in its position as a result of influences during coating are prevented. Also, it is impossible to prevent oscillations in the rod during coating.
One object of the present invention, therefore, is to provide an improved process so as to obtain a relatively constant layer thickness profile along the coating region, without the disadvantages of the prior art.
Another object of the invention is to provide an article of manufacture, namely a tubular glass blank for conducting the process, as well as the resultant coated preform.
Upon further study of the specification and appended claims, other objects and advantages of this invention will become apparent to those skilled in the art.